The present invention is related to an antenna circuit for effectively transferring an antenna output signal outputted from an antenna to a receiver.
In FIG. 4, there is shown an example of a conventional antenna circuit provided in a base terminal unit of an AM/FM receiving antenna. In FIG. 4, an antenna output signal of an AM/FM receiving antenna 10 is subdivided into two antenna output signals. One antenna output signal is supplied to an FM band filter 12, an output signal derived from this FM band filter 12 is supplied to an FM amplifying circuit 14, and furthermore, an amplified FM signal outputted from the FM amplifying circuit 14 is supplied to an FM receiver 16. Also, the other divided antenna output signal is supplied to an AM band filter 18, an antenna output signal outputted from this AM band filter 18 is supplied to an AM amplifying circuit 20, and furthermore, an amplified AM signal outputted from the AM amplifying circuit 20 is supplied to an AM receiver 22. These FM band filter 12, FM amplifying circuit 14, AM band filter 18, and AM amplifying circuit 20 are provided in either the base terminal unit of the AM/FM receiving antenna 10 or the unit near this AM/FM receiving antenna 10. The amplified output signals of the FM amplifying circuit 14 and the AM amplifying circuit 20 are transferred via a cable to both the FM receiver 16 and the AM receiver 22 in a proper manner, respectively.
In the FM band filter 12, an input terminal to which the antenna output signal is supplied is connected to an output terminal via a series-connected circuit made by sequentially connecting a capacitor Cl, two coils L1, L2, and another capacitor C2 in a serial manner. Then, the FM band filter 12 is arranged by connecting a junction point between the two coils L1 and L2 via a parallel circuit of a capacitor C3 and a coil L3 to the ground.
Also, the FM amplifying circuit 14 to which the FM band signal filtered through the FM band filter 12 is supplied is arranged as follows: In other words, an input terminal is connected via a capacitor C4 to a base of a transistor Tr1. This base is grounded via a resistor R1, and also is connected via a resistor R2 to one terminal of a parallel-connected circuit made of a resistor R3 and a capacitor C5. The other terminal of this parallel-connected circuit is connected to a collector of the transistor Tr1. This collector is connected via a capacitor C6 to the output terminal. Also, this collector is connected via a series-connected circuit of a coil L4 and a resistor R4 to a power supply terminal xe2x80x9c+Bxe2x80x9d. A junction point between this coil L4 and a resistor R4 is grounded via a capacitor C7. Furthermore, an emitter of the transistor Tr1 is grounded via a parallel-connected circuit made of a resistor R5 and a capacitor C8.
Furthermore, the AM band filter 18 is so arranged that an input terminal to which the antenna output signal is supplied is connected via a coil L5 to an output terminal, and this output terminal is grounded via a capacitor C9.
In this arrangement, the FM band filter 12 may block the AM band signal and may pass the FM band signal therethrough by properly set the constants of the circuit structural elements. Then, the FM amplifying circuit 14 constitutes the emitter grounded type transistor circuit in which the radio (high) frequency load is provided at the collector of the transistor Tr1, and is operable as a voltage amplification type amplifying circuit. As a result, the FM band signal which has passed through the FM band filter 12 is voltage-amplified by the voltage amplification effect of this transistor Tr1, and then, the voltage-amplified FM band signal is outputted. Also, in the circuit arrangement of this FM amplifying circuit 14, the input impedance is made substantially equal to the output impedance, and the output impedance of the AM/FM receiving antenna 10 via the FM band filter 12 is matched with the input impedance of the FM receiver 16.
In this arrangement, the AM band filter 18 may block the FM band signal and may pass the AM band signal therethrough by properly set the constants of the circuit structural elements. Only the AM band signal is amplified by the AM amplifying circuit 20 to output the amplified AM band signal.
On the other hand, in the above-explained conventional FM amplifying circuit 14, the voltage of the FM band signal functioning as the input signal to be amplified is applied between the base of the transistor Tr1 and the emitter thereof, and furthermore, the voltage of the amplified output signal is applied between the collector of the transistor Tr1 and the emitter thereof. Then, when both the input signal and the output signal are increased, the amplification characteristic defined by the amplifying capability and the bias condition of the transistor Tr1 exceeds a pre-selected range indicative of the linearity, so that high frequency (radio frequency) distortions and cross modulation distortions would occur. Then, this input/output signal amplitude characteristic is indicated by a circle symbol in FIG. 2 under such a condition that the amplitude of the output signal is saturated with respect to the increase of the amplitude of the input signal. Accordingly, there is such a problem that the cross modulation distortion is strengthened under such high electric fields as a place near an FM broadcasting signal transmission station, and therefore, radio interference would occur.
Also, as shown in FIG. 5(a), assuming now that as FM band signals of high electric fields, for example, two broadcasting waves 1 and 2 having frequencies of 76 MHz and 76.6 MHz are present, these broadcasting waves 1 and 2 are applied to the AM amplifying circuit 20 without being sufficiently attenuated/blocked by the AM band filter 18. As a result, as shown in FIG. 5(b), the second-order cross modulation wave of 600 KHz equal to a frequency difference between the two broadcasting waves 1 and 2 is also amplified by the AM amplifying circuit 20 to output the amplified second-order cross modulation wave. This 600 KHz signal is contained in the frequency range of the AM band signal, and may function as the interference wave in the AM receiver 22.
Furthermore, conventionally, as the AM/FM receiving antenna 10, such an antenna having a physical length of approximately 1 m is generally used. Thus, the antenna output signal having the practically high level with respect to the AM band signal could be obtained from this general-purpose antenna. On the other hand, since a strong demand is recently made of compact antennas, there is such a trend that as to the AM/FM receiving antenna 10, a shorter antenna is used by employing a helical coil and the like, while the antenna effective length is equal to approximately 1 m with respect to the FM band signal, but the antenna physical length is made shorter than, or equal to 50 cm. Thus, although the antenna output signal for the FM band signal could be obtained at the substantially same level as that of the conventional antenna, the level of the AM band signal would be largely reduced. As a result, this may cause a serious problem. That is, the attenuation of the AM band signal by the FM band filter 12 cannot be neglected. This is because the coils L1 and L3 which constitute the FM band filter 12 own the small impedances with respect to the AM band signal, and the AM band signal is largely attenuated via the capacitor C1, as represented in the equivalent circuit diagram shown in FIG. 3(b). Thus, since the AM band signal supplied the AM band filter 18 is reduced, there is such a problem that the reception sensitivity with respect to the AM band signal would be largely deteriorated.
The present invention has an object to provide an antenna circuit capable of improving deterioration of an amplification characteristic thereof under high electric field. Also, the present invention has an object to provide an AM receiving antenna circuit capable of firmly blocking/attenuating an FM band signal of a high electric field, and also capable of passing the AM band signal without attenuating this AM band signal, and furthermore capable of outputting the amplified AM band signal. Moreover, the present Invention has an object to provide an AM/FM receiving antenna circuit by which even when an antenna having a short dimension is employed, an AM band signal is not attenuated by an FM band filter, and an AM band signal is supplied to an AM band filter while maintaining a practically high signal level.
To achieve the objects, an antenna circuit, according to the present invention, is arranged by that an antenna output signal is applied to an input terminal of a current amplification type amplifier having a high input impedance and also a low input impedance; an output terminal of the current amplification type amplifier is connected to an input terminal of a band-pass filter; and a signal is outputted from an output terminal of this band-pass filter to a receiver, and the above-described band-pass filter may pass therethrough all band signals of the reception frequencies of the receiver. With employment of the above-described arrangement, an open-output-signal voltage of an AM/FM receiving antenna can be directly applied to the input terminal of the FM receiver without being attenuated, so that a high gain can be obtained. Moreover, the distortion characteristic in the input/output signal amplitude characteristic becomes superior.
Also, an AM receiving antenna circuit, according to the present invention, is featured by such an AM receiving antenna circuit comprising: an AM band filter for blocking an FM band signal contained in an antenna output signal and also for passing an AM band signal therethrough; and an AM amplifying circuit for amplifying the filtered AM band signal to output the amplified AM band signal, wherein: the AM band filter is arranged in such a manner that an input terminal to which the antenna output signal is supplied is connected via a series circuit made of a first coil and a second coil to an AM signal input terminal of the AM amplifying circuit, and a junction point between the first coil and second coil is grounded via a series circuit made of a capacitor and a third coil; and the AM amplifying circuit is arranged in such a manner that a field-effect transistor is included, the AM signal input terminal is connected to a gate of the field-effect transistor, a source thereof is grounded; a first filter for blocking an FM band signal is formed by the first coil and the capacitor; a trap circuit for attenuating the FM band signal is formed by the capacitor and the third coil; and a second filter for blocking the FM band signal is formed by the second coil and an input capacitance formed between the gate of the filed-effect transistor and the source thereof. With employment of the above-explained arrangement, the first filter and the second filter, which may block the FM band signal, the second filter, and the trap circuit are formed, the FM band signal is sufficiently attenuated/blocked in plural stages, and thus, only the AM band signal is supplied to the AM amplifying circuit.
Then, an AM/FM receiving antenna circuit, according to the present invention, is featured by such an AM/FM receiving antenna circuit for subdividing an antenna output signal into two antenna output signals and for supplying these two antenna signals to an FM band filter for blocking an AM band signal and also for passing an FM band signal therethrough and also to an AM band filter for blocking the FM band signal and also for passing the AM band signal therethrough, wherein: one of the subdivided antenna output signal is first supplied to a capacitor, an output of this capacitor is supplied to a coil, and then this subdivided antenna signal is supplied to the FM band filter via an FM series resonant circuit constituted by the above-explained capacitor and coil. With employment of the above-described arrangement, even when the impedance of the coil of the series resonant circuit and the impedance of the coil of the FM band filter are small with respect to the AM band signal, the AM band signal is attenuated via the series circuit made of the capacitor of the FM series resonant circuit and the capacitor of the FM band filter. However, this AM band signal is not largely attenuated by the FM band filter similar to the prior art.